Reference Signal for a Coordinated Multi-Point Network Implementation

ABSTRACT

A system and method for broadcasting a channel state information reference signal (CSI-RS) is disclosed. A CSI-RS that is orthogonal to CSI-RSs transmitted by each of a first network cell and each of a set of neighbor cells that interfere with the first network cell is identified. In one implementation, the first network cell has a coverage containing a coverage of a second network cell. The method includes transmitting, from the second network cell, the CSI-RS that is orthogonal to CSI-RSs transmitted by each of the first network cell and each of a set of interfering neighbor cells.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. provisional patent applicationNo. 61/304,364 entitled “REFERENCE SIGNAL FOR A COORDINATED MULTI-POINTNETWORK IMPLEMENTATION” and filed on Feb. 12, 2010.

BACKGROUND

The present invention relates generally to data transmission in mobilecommunication systems and more specifically to a channel stateinformation (CSI) reference signal (RS) to support coordinatedmulti-point network implementations and heterogeneous networks.

As used herein, the terms “user equipment” and “UE” can refer towireless devices such as mobile telephones, personal digital assistants(PDAs), handheld or laptop computers, and similar devices or other UserAgents (“UAs”) that have telecommunications capabilities. A UE may referto a mobile, or wireless device. The term “UE” may also refer to devicesthat have similar capabilities but that are not generally transportable,such as desktop computers, set-top boxes, or network nodes.

In traditional wireless telecommunications systems, transmissionequipment in a base station transmits signals throughout a geographicalregion known as a cell. As technology has evolved, more advancedequipment has been introduced that can provide services that were notpossible previously. This advanced equipment might include, for example,an evolved universal terrestrial radio access network (E-UTRAN) node B(eNB) rather than a base station or other systems and devices that aremore highly evolved than the equivalent equipment in a traditionalwireless telecommunications system. Such advanced or next generationequipment may be referred to herein as long-term evolution (LTE)equipment, and a packet-based network that uses such equipment can bereferred to as an evolved packet system (EPS). Additional improvementsto LTE systems/equipment will eventually result in an LTE advanced(LTE-A) system. As used herein, the phrase “base station” or “accessdevice” will refer to any component, such as a traditional base stationor an LTE or LTE-A base station (including eNBs), that can provide a UEwith access to other components in a telecommunications system.

In mobile communication systems such as the E-UTRAN, a base stationprovides radio access to one or more UEs. The base station comprises apacket scheduler for dynamically scheduling downlink traffic data packettransmissions and allocating uplink traffic data packet transmissionresources among all the UEs communicating with the base station. Thefunctions of the scheduler include, among others, dividing the availableair interface capacity between UEs, deciding the transport channel to beused for each UE's packet data transmissions, and monitoring packetallocation and system load. The scheduler dynamically allocatesresources for Physical Downlink Shared CHannel (PDSCH) and PhysicalUplink Shared CHannel (PUSCH) data transmissions, and sends schedulinginformation to the UEs through a scheduling channel.

It is generally desirable to provide a high data rate coverage usingsignals that have a high Signal to Interference Plus Noise ratio (SINR)for UEs serviced by a base station. Typically, only those UEs that arephysically close to a base station can operate with a very high datarate. Also, to provide high data rate coverage over a large geographicalarea at a satisfactory SINR, a large number of base stations aregenerally required. As the cost of implementing such a system can beprohibitive, research is being conducted on alternative techniques toprovide wide area, high data rate service.

Coordinated multi-point (CoMP) transmission and reception may be used toincrease transmission data rate and/or signal quality in wirelesscommunication networks such as LTE-A networks. Using CoMP, neighboringbase stations coordinate to improve the user throughput or signalquality, especially for users at a cell edge. CoMP may be implementedusing a combination of base stations such as eNBs, and/or relay nodes(RN) and/or other types of network nodes and/or cells.

FIG. 1 is an illustration of a wireless communications network havingtwo eNBs operating in a CoMP transmission and reception configuration. Asimilar illustration can be applied to a combination of eNBs, RNs and/orcells. As illustrated in FIG. 1, in network coverage area 104, eNBs 106and 108 are configured to transmit communication signals to UE 110. Innetwork coverage area 104, any collaboration scheme may be used for eNBs106 and 108. For example, in some CoMP schemes, eNB 106 and eNB 108 maywork together to transmit the same signal to UE 110 at the same time. Insuch a system, the signals transmitted by the base stations combine(i.e., superpose) in the air to provide a stronger signal and thusincrease the chance of transmission success. In other CoMP schemes, eNB106 and eNB 108 transmit different signals to UE 110, which, forexample, include different data that is to be communicated to UE 110. Bytransmitting different portions of the data through different eNBs, thethroughput to UE 110 may be increased. The use of CoMP depends on manyfactors including channel conditions at UE 110, available resources,Quality of Service (QoS) requirements, etc. As such, in some networkimplementations, in a given node/cell or combination of nodes/cells onlya subset of available UEs may be serviced with CoMP transmissions. Forexample, in FIG. 1, UE 112 is only served by eNB 108.

In LTE-A, CoMP can be used to improve the throughput for cell edge UEsas well as the cell average throughput. There are two primary mechanismsby which CoMP transmissions may be implemented to recognize theseimprovements. First, CoMP transmissions may provide coordinatedscheduling, where data is transmitted to a single UE from one of theavailable transmission points (e.g., one of the available eNBs in FIG. 1or one of the available network nodes or cells) and scheduling decisionsare coordinated to control, for example, the interference generated in aset of coordinated cells. Secondly, CoMP transmissions may provide jointprocessing where data is simultaneously transmitted to a single UE frommultiple transmission points, for example, to (coherently ornon-coherently) improve the received signal quality and/or activelycancel interference for other UEs.

In the case of coordinated scheduling, data is only transmitted by theserving cell, but the scheduling decisions are made with coordinationamong the neighboring cells. In the case of joint processing CoMPtransmission, multiple base stations transmit the data to the same usersimultaneously. The UE then jointly processes the transmissions frommultiple nodes to achieve a performance gain.

In CoMP implementations, the serving cell may be the cell transmittingPhysical Downlink Control Channel (PDCCH) assignments (i.e., a singlecell). This is analogous to the serving cell of Rel-8. In CoMP, dynamiccell selection involves a PDSCH transmission from one point within theCoMP cooperating set at a first time and in CoordinatedScheduling/Beamforming (CS/CB) data is only available at the servingcell (data transmission from that point) but user scheduling/beamformingdecisions are made with coordination among cells corresponding to theCoMP cooperating set.

When implementing CoMP, a series of CoMP cell sets may be defined. In aCoMP cooperating set, a set of (geographically separated) pointsdirectly or indirectly participate in PDSCH transmission to the UE. Thecooperating set may be transparent to the UE. CoMP transmission point(s)are a point or set of points actively transmitting PDSCH to the UE. CoMPtransmission point(s) are a subset of the CoMP cooperating set. Forjoint transmission, the CoMP transmission points are the points in theCoMP cooperating set, but for dynamic cell selection, a single point isthe transmission point at each subframe. The transmission point canchange dynamically within the CoMP cooperating set. A CoMP measurementset is a set of cells about which channel state/statistical information(CSI) related to their link to the UE is reported. The CoMP measurementset may be the same as the CoMP cooperating set. A Radio ResourceMeasurement (RRM) measurement set is a set in support of RRMmeasurements that may be defined in Rel-8 and is, therefore, notCoMP-specific. For Coordinated scheduling/beamforming, the CoMPtransmission point may correspond to the “serving cell.”

In LTE systems, data is transmitted from an access device to UEs viaResource Blocks (RBs). Referring to FIG. 2, an exemplary resource block50 is illustrated that is comprised of 168 Resource Elements (REs) (seeexemplary elements 52) arranged in twelve frequency columns and fourteentime rows as known in the art. Accordingly, each element corresponds toa different time/frequency combination. The combination of elements ineach time row are referred to as an Orthogonal Frequency DivisionMultiplexing (OFDM) symbol. In the illustrated example the first threeOFDM symbols (in some cases it may be the first two, first four, etc.)are reserved for PDCCH 56 and are shown in FIG. 2 as gray REscollectively. Various types of data can be communicated in each RE.

LTE systems employ various types of reference signals to facilitatecommunication between an access device or base station and a UE. Areference signal can be used for several purposes including determiningwhich of several different communication modes should be used tocommunicate with UEs, channel estimation, coherent demodulation, channelquality measurement, signal strength measurements, etc. Referencesignals are generated based on data known to both an access device and aUE, and may also be referred to as pilot, preamble, training signals, orsounding signals. Exemplary reference signals include a cell specificreference signal (CRS) that is sent by a base station to UEs within acell and is used for channel estimation and channel quality measurement,a UE-specific or dedicated reference signal (DRS) that is sent by a basestation to a specific UE within a cell that is used for demodulation ofa downlink, a sounding reference signal (SRS) sent by a UE that is usedby a base station for channel estimation and channel quality measurementand a demodulation reference signal sent (DM-RS) by a UE that is used bya base station for channel estimation of an uplink transmission from theUE.

In LTE systems, CRSs and DRSs are transmitted by base stations in RBREs. To this end, see FIG. 2 which shows an exemplary CRS (three ofwhich are labeled 52) in vertical, horizontal, left down to right andleft up to right hatching for ports 0 through 3 respectively andexemplary DRS in dark REs to the right of the three columns of PDCCH 56,three of which are labeled 54. The reference signals allow any UEscommunicating with the access device to determine channelcharacteristics and to attempt to compensate for poor characteristics.The CRSs are UE-independent (i.e., are not specifically encoded forparticular UEs) and, in at least some cases, are included in all RBs. Bycomparing the received CRS to known reference signals (i.e., knowndata), a UE can determine channel characteristics (e.g., a channelquality information, etc.). The difference between the known data andthe received signal may be indicative of signal attenuation, path-lossdifferences, etc.

UEs report channel characteristics back to the base station and the basestation then modifies its output (i.e., subsequent REs) to compensatefor the channel characteristics. To indicate how signal output ismodified, the base station transmits a UE-specific DRS to each UE. Hereagain, DRS data is known at the UE and therefore, by analyzing receivedDRS the UE can determine how the access device output has been modifiedand hence obtain information required to demodulate data received insubsequent REs. In FIG. 2, exemplary CRS reference signals are indicatedby hatching, DRS signals are indicated by dark REs and non-referencesignal elements during which traffic data is transmitted are blank(i.e., white).

Referring again to FIG. 2, to avoid collisions, LTE system DRS 54 aregenerally allocated to OFDM symbols separate from those occupied by CRS.Furthermore, DRS 54 are generally allocated away from PDCCH 56. Inrelease 8 LTE devices (hereinafter “Rel-8 devices”), for example, DRS ofantenna port 5 may be specified for PDSCH demodulation as shown in FIG.2. In some cases, CRS 52 on antenna ports 0-3 are distributed on all RBsin the system bandwidth, while DRS 54 on antenna port 5, for example,may only be allocated in RBs assigned to a corresponding UE. When a UEis assigned two or more contiguous RBs, DRS 54 allocation may simply berepeated from one RB 50 to the next.

Two new types of reference signals are defined in LTE-A for the purposeof channel estimation for demodulation: channel estimation for channelstate information (CSI) measurement and channel quality indicator (CQI)measurement. The first type of RS is a UE-specific RS or UE-RS used fordemodulation of the traffic channel assigned to the UE, i.e. thephysical downlink shared channel (PDSCH). The UE-RS is also calleddemodulation RS (DM-RS). The second type of RS is a cell-specific RSused for CSI measurement and CQI measurement. In LTE-A, the LTE Rel-8common reference signal (CRS) may be retained in non-Multicast/Broadcastover a Single Frequency Network (MBSFN) subframes to support legacyRel-8 UEs. In an MBSFN subframe which may be used as a subframe to onlysupport LTE-A UE, CRS may only be retained within the PDCCH region.

In some network implementations, then anticipated CSI-RS overhead isapproximately 1/840=0.12% per antenna port (8 antenna ports=0.96%). Forexample, CSI-RS may be implemented with a time density of 1 symbol every10 ms per antenna port: 1/140, or a frequency density of 1 subcarrierevery 6 subcarriers per antenna port: 1/6. The periodicity of the CSI-RSsignal may be adjusted by an integer number of timeframes. For DM-RS thebroadcast rate is: Rank 1 transmission—12 REs per RB (same overhead asRel-8); Rank 2 transmission—12 REs per RB to be confirmed, and Rank 3-8transmissions—a maximum of 24 REs (total) per RB. Generally, the sameREs per antenna port are transmitted for each DM-RS rank.

There are several difficulties associated with current CSI-RS designs.First, to support CoMP multi-cell CSI measurement at the UE, the UE isrequired to detect the CSI-RS transmitted by neighboring cells with asufficient level of accuracy. However, because the signal strengthreceived from neighboring cells can be relatively low compared to thesignal strength received from the serving cell and the sum of the signalstrength received from other neighboring cells, the received SINR of aneighboring cell CSI-RS can be quite low.

Also, existing CSI-RS design focuses on a homogeneous network scenariowhere only macro cells are deployed. Future networks, however, may beimplemented using heterogeneous networks incorporating macro cellsoverlaid with small cells (also called low power nodes, e.g. femto cell,relay cell, pico cell etc.). In that case, the expected reuse clustersize will need to a much larger than the 6 to 8 cluster size currentlyspecified. Because macro eNBs and small cell eNBs have very differenttransmit power (the transmit power of a macro eNB is 46 dBm (for 10 MHzbandwidth) whereas the transmit power of a pico eNB, femto eNB and relaynode (RN) is 30 dBm, 20 dBm and 30 dBm respectively for 10 MHzbandwidth), the larger transmit power of the macro eNB will lead tosevere DL interference experienced by a UE attached to the low powernode that is located within the macro eNB coverage. This severeouter-cell interference will be detrimental to the performance ofcontrol channels (e.g. PDCCH), data channels (e.g. PDSCH) and RSdetection, including CSI-RS detection.

Finally, to support CoMP with higher reuse cluster sizes andmulti-antenna configurations, the number of CSI-RS antenna ports will besignificant. To limit overhead, a larger periodicity of the CSI-RS maybe required. A larger interval between CSI-RS transmissions maynegatively affect detection performance of CSI-RS for a higher speedmobile that may, or may not, be in CoMP operation.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following brief description, taken in connection with theaccompanying drawings and detailed description, wherein like referencenumerals represent like parts.

FIG. 1 is an illustration of a wireless communications network havingtwo eNBs operating in a coordinated multi-point (CoMP) transmission andreception configuration;

FIG. 2 illustrates a resource block (RB) including both CRS and aplurality of Dedicated Reference Signals (DRSs) distributed throughoutthe RB;

FIG. 3 is an illustration that shows two example orthogonal CSI-RStransmissions broadcast from first and second neighbor cells, where eachCSI-RS transmission includes PDSCH RE muting;

FIG. 4 is an illustration of an example of an RB having REs availablefor CSI-RS transmission, the REs are selected based upon severalconditions;

FIG. 5 is an illustration of an example network mapping showing manycells, with a subset of the cells being arranged in a CSI-RS group;

FIG. 6 is an illustration showing reservation of available REs in an RBfor CSI-RS ports within a CSI-RS group;

FIGS. 7A-7C illustrate 3 CSI-RS groups where different sets of mutuallyexclusive (or orthogonal) CSI-RS port resources reserved for differentCSI-RS groups are provided using TDM;

FIG. 8 is an illustration of multiplexing of mutual exclusive sets ofCSI-RS port resources for different CSI-RS groups within a single RB;

FIGS. 9A and 9A are illustrations of the CSI-RS port resources mappingof a first CSI-RS group over time;

FIGS. 10A-10C are illustrations of different or mutually orthogonalCSI-RS port resources reserved for different CSI-RS groups where 8CSI-RS port resources (i.e. 16 REs) are reserved for each CSI-RS groupin each of the three subframes X, Y, and Z;

FIG. 11 is an illustration of ordering of CSI-RS port resources reservedfor a CSI-RS group and indexing each CSI-RS port resource with a logicalindex;

FIG. 12 is an illustration of available REs in an RB for CSI-RS in aMBSFN subframe;

FIG. 13 is an illustration of a CSI-RS grouping within a network showingthe strongest neighbor cells to UEs at different location within thecell;

FIG. 14 is an illustration of RBs based on PDSCH REs mutingrequirements, the RBs within each RB group can be contiguous ornon-contiguous;

FIG. 15 is an illustration of an example network including several macrocells with small cells #1, #2, and #3 disposed within the macro cells;

FIG. 16 is an illustration of an alternative small cell networkdeployment where one or more of the small cells overlap;

FIG. 17 is an illustration of a network implementation including anoverlay of small cells on top of macro cell coverage where, in somecases, the coverage of the small cells overlaps;

FIG. 18 is an illustration of interleaved normal and supplemental CSI-RSsubframe locations, each having a periodicity of 10 subframes (or oneframe);

FIG. 19 is a diagram of a wireless communications system including a UEoperable for some of the various embodiments of the disclosure;

FIG. 20 is a block diagram of a UE operable for some of the variousembodiments of the disclosure;

FIG. 21 is a diagram of a software environment that may be implementedon a UE operable for some of the various embodiments of the disclosure;and

FIG. 22 is an illustrative general purpose computer system suitable forsome of the various embodiments of the disclosure.

DETAILED DESCRIPTION

The present invention relates generally to data transmission in mobilecommunication systems and more specifically to a channel stateinformation (CSI) reference signal (RS) to support coordinatedmulti-point (COMP) network implementations and heterogeneous networks.

Some implementations include a method of decoding a channel stateinformation reference signal (CSI-RS) using a user equipment (UE). Themethod includes receiving an indication of a resource element (RE)configuration allocated for transmission of CSI-RSs by a first cell. Theindication is received from a second cell. The method includes at leastone of using the indication of the RE configuration to decode a firstCSI-RS received from the first cell, and using the indication of the REconfiguration to mute one or more REs within a data channel transmissionreceived from a third cell. The first cell, second cell and third cellmay be associated within a CSI-RS group. At least two of the first cell,the second cell, and the third cell may be mutually interfering cells.

Other implementations include a method of transmitting a channel stateinformation reference signal (CSI-RS) to a user equipment (UE). Themethod includes providing a first resource block (RB) configuration forat least one UE experiencing interference from a first set ofinterfering neighbor cells, providing a second RB configuration for atleast one UE experiencing interference from a second set of interferingneighbor cells, and receiving a measurement report from a first UE. Themeasurement report identifies a set of interfering neighbor cells forthe first UE. The method includes, when the set of interfering neighborcells for the first UE is included within the first set of interferingneighbor cells, transmitting the first RB configuration to the first UE,and, when the set of interfering neighbor cells for the first UE isincluded within the second set of interfering neighbor cells,transmitting the second RB configuration to the first UE.

Other implementations include a method of receiving a channel stateinformation reference signal (CSI-RS). The method includes transmittinga measurement report to a first cell. The measurement report identifiesa set of interfering neighbor cells for the UE. The method includesreceiving a resource block (RB) configuration from the first cell, andusing the RB configuration to at least one of decode a CSI-RS receivedfrom an interfering cell and mute at least one resource element (RE)within a data channel transmission received from a second interferingcell.

Other implementations include a user equipment (UE) comprising aprocessor configured to receive an indication of a resource element (RE)configuration allocated for transmission of CSI-RSs by a first cell. Theindication is received from a second cell. The processor is configuredto at least one of use the indication of the RE configuration to decodea first CSI-RS received from the first cell, and use the indication ofthe RE configuration to mute one or more REs within a data channeltransmission received from a third cell. The first cell, second cell andthird cell may be associated within a CSI-RS group. At least two of thefirst cell, the second cell, and the third cell may be mutuallyinterfering cells.

Other implementations include a base station comprising a processorconfigured to identify a first resource block (RB) configuration for atleast one UE experiencing interference from a first set of interferingneighbor cells, identify a second RB configuration for at least one UEexperiencing interference from a second set of interfering neighborcells, and receive a measurement report from a first UE. The measurementreport identifies a set of interfering neighbor cells for the first UE.The processor is configured to, when the set of interfering neighborcells for the first UE is included within the first set of interferingneighbor cells, transmit the first RB configuration to the first UE,and, when the set of interfering neighbor cells for the first UE isincluded within the second set of interfering neighbor cells, transmitthe second RB configuration to the first UE.

To the accomplishment of the foregoing and related ends, the invention,then, comprises the features hereinafter fully described. The followingdescription and the annexed drawings set forth in detail certainillustrative aspects of the invention. However, these aspects areindicative of but a few of the various ways in which the principles ofthe invention can be employed. Other aspects, advantages and novelfeatures of the invention will become apparent from the followingdetailed description of the invention when considered in conjunctionwith the drawings.

The various aspects of the subject invention are now described withreference to the annexed drawings, wherein like numerals refer to likeor corresponding elements throughout. It should be understood, however,that the drawings and detailed description relating thereto are notintended to limit the claimed subject matter to the particular formdisclosed. Rather, the intention is to cover all modifications,equivalents, and alternatives falling within the spirit and scope of theclaimed subject matter.

As used herein, the terms “component,” “system” and the like areintended to refer to a computer-related entity, either hardware, acombination of hardware and software, software, or software inexecution. For example, a component may be, but is not limited to being,a process running on a processor, a processor, an object, an executable,a thread of execution, a program, and/or a computer. By way ofillustration, both an application running on a computer and the computercan be a component. One or more components may reside within a processand/or thread of execution and a component may be localized on onecomputer and/or distributed between two or more computers.

The word “exemplary” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other aspects or designs.

Furthermore, the disclosed subject matter may be implemented as asystem, method, apparatus, or article of manufacture using standardprogramming and/or engineering techniques to produce software, firmware,hardware, or any combination thereof to control a computer or processorbased device to implement aspects detailed herein. The term “article ofmanufacture” (or alternatively, “computer program product”) as usedherein is intended to encompass a computer program accessible from anycomputer-readable device, carrier, or media. For example, computerreadable media can include but are not limited to magnetic storagedevices (e.g., hard disk, floppy disk, magnetic strips . . . ), opticaldisks (e.g., compact disk (CD), digital versatile disk (DVD) . . . ),smart cards, and flash memory devices (e.g., card, stick). Additionallyit should be appreciated that a carrier wave can be employed to carrycomputer-readable electronic data such as those used in transmitting andreceiving electronic mail or in accessing a network such as the Internetor a local area network (LAN). Of course, those skilled in the art willrecognize many modifications may be made to this configuration withoutdeparting from the scope or spirit of the claimed subject matter.

In network implementations that include several broadcasting neighboringcells, it may be difficult to receive and distinguish CSI-RSstransmitted by each of the neighboring cells. In some cases, the signalstrength from the neighboring cells is relatively low compared to thesignal strength from the serving cell. Also, the signal strength of asingle neighboring cell is relatively low when compared to the sum ofsignals received from the other neighboring cells and the serving cell.To address these problems, in the present system and method, each of theneighboring cells may be configured to broadcast CSI-RSs using REs thatare not in use by the other neighboring cells within the reuse clusterfor CSI-RS transmission. For example, in a first neighbor cell, PDSCHREs that coincide with the CSI-RS transmitted by neighbor cells withinthe reuse cluster may be muted (e.g., not used) so that the REs do notinterfere with one another. This may improve the neighbor cell CSI-RSdetection and channel estimation accuracy to support CoMP transmission(e.g. joint processing (JP), coordinated beamforming (CB), etc).

FIG. 3 is an illustration that shows two example orthogonal CSI-RStransmissions broadcast from first and second neighbor cells, and mutingof PDSCH REs in the first and second neighbor cells to avoid collisionwith the CSI-RS transmissions from each other and from other neighborcells. By muting certain REs within each RB, interference to the CSI-RSsbroadcast by each of Cell #0 and Cell #1 are minimized. With referenceto FIG. 3, each of Cell #0 and Cell #1 use two CSI-RS antenna portswhere each CSI-RS port transmits on two REs (see the pairs of REslabeled 70 and 72 in each CSI-RS for cell #0 and cell #1). To avoidinterference between the CSI-RSs transmitted by each cell, theorthogonality of the CSI-RSs is maintained through time divisionmultiplexing (TDM) and/or frequency division multiplexing (FDM) of REswithin the RB. As shown in FIG. 3, the CSI-RS REs for cell #0 are offsetby one subcarrier from the CSI-RS REs for cell #1. Furthermore, severalPDSCH REs are muted, to minimize interference to CSI-RSs transmitted byother neighbor cells. In other words, the PDSCH REs that coincide withthe CSI-RS REs transmitted by neighbor cells may be muted.

In the illustration of FIG. 3, there are a total of 16 REs that can beused within an RB for CSI-RS transmission or PDSCH RE muting.Accordingly, in this configuration, up to four different neighbor cellscan transmit CSI-RSs using the RB configuration shown in FIG. 3 whereinthe CSI-RS from each cell will not interfere with one another (as theCSI-RS REs of a single cell only overlap with muted REs broadcast by theremaining cells). Accordingly, the illustrated configuration supports upto 4 cells within the CSI-RS reuse cluster. To minimize the effect onRel-8 UE PDSCH reception, it may be recommended that the number of mutedor punctured REs per RB should be no more than 16, 24 or 32.

For a given CSI-RS configuration, the reuse factor indicates the numberof neighbor cells that can transmit mutually orthogonal CSI-RSs. Theorthogonality of CSI-RSs can be achieved by different cells transmittingCSI-RSs on different time/frequency tones or REs. The reuse factor forthe CSI-RS on each subframe may be dependent on the maximum allowablenumber of muted/punctured REs per RB, the number of REs per CSI-RSantenna port per RB, the number of CSI-RS antenna ports (or transmitantennas) per cell. Table 1 illustrates different reuse factorsresulting from different values of the number of REs per CSI-RS antennaport, and the number of CSI-RS antenna ports (or transmit antennas) percell. It can be seen that if a CSI-RS is transmitted by each cell inevery subframe, the reuse factor in some cases is not sufficient tosupport a homogeneous network and in all cases are not sufficient tosupport a heterogeneous network.

TABLE 1 Maximum allowable Number of REs per Number of CSI-RS antennaReuse factor of CSI-RS number of muted/ CSI-RS antenna ports per cell(i.e. number per subframe, R = punctured REs per RB, port per RB, oftransmit antennas N_(muted+punctured)/ N_(muted+punctured) N_(RE) _(—)_(per) _(—) _(antenna) per cell), N_(tx) (N_(RE) _(—) _(per) _(—)_(antenna) × N_(tx)) 16 2 2 4 16 2 4 2 16 2 8 1 16 1 2 8 16 1 4 4 16 1 82

A reuse cluster of neighbor cells, as described above does not take intoaccount of the possibility of CSI-RS hopping to further randomize theCSI-RS collision and interference and does not differentiate between themaximum allowable number of muted/punctured REs per RB and the number ofavailable resources of REs per RB that can be used for CSI-RStransmission. In some cases, the number of available resources of REsper RB that can be used for CSI-RS transmission can be much larger thanthe maximum allowable number of muted/punctured REs per RB.

Furthermore, the muting approach described above does not consider orcompensate for a mixture of non-CoMP and CoMP network operations withina particular cell. Also, possible muting, TDM, and FDM approaches do notscale to support the high density deployment of small cell nodes andoverlaid nature of a heterogeneous network.

CSI-RS hopping over time may randomize CSI-RS collisions amongneighboring cells in the case of non-CoMP network implementations. FIG.4 is an illustration of an example of an RB having REs available forCSI-RS transmission. The REs are selected based upon the followingconditions: 1) the CSI-RS cannot puncture the PDCCH region, i.e. thefirst 3 OFDM symbols labeled 200; and 2) the CSI-RS cannot puncture theRel-8 CRS and the Rel-9/Rel-10 DM-RS. In an RB there are 52 availableREs for CSI-RS transmission assuming that the RE pair used for eachCSI-RS port are 6 sub-carriers apart. As each CSI-RS port requires twoREs, the total number of possible CSI-RS port resources is 26.

In one example implementation, a cell requires 4 CSI-RS ports, thus 8 ofthe available REs. In that case, the typical number of required REs forCSI-RS transmission (i.e., 8 CSI-RS REs as shown in this example orlimited to 16, 24, 32 to avoid too much puncturing to Rel-8 PDSCHperformance) is less than the total available REs for CSI-RStransmission (i.e., 52 in this case). Therefore, the mapping of CSI-RSports to the available REs can hop over time and be randomized acrossneighbor cells. This provides randomization of CSI-RS collisions acrossneighbor cells, minimizing the inter-cell CSI-RS interference.

For heterogeneous networks, even without support of CoMP, however, inthe case of severe interference caused by macro base stations to smallcells, random hopping does not guarantee full collision avoidance andmay be insufficient. Also, in the case of a CoMP network implementation,the described hopping may not ensure that the CSI-RS of differentneighboring cells within the reuse cluster do not collide with oneanother.

In some cases, the CSI-RS ports (time/frequency locations) used bydifferent cells and the hopping pattern may be defined based on cell ID.For CoMP, however, to maintain full orthogonality of CSI-RS among cellswithin the reuse cluster, cell ID-based CSI-RS ports allocation andhopping are not suitable. Similarly, for heterogeneous networks, toavoid severe interference from PDSCH transmissions and CSI-RStransmissions generated by macro cells with the CSI-RS of a small cell,the CSI-RS ports allocation to both macro cells and small cells need tobe carefully planned and not randomized simply by cell ID.

In the present system, various CSI-RS groups may be defined, with eachCSI-RS group including a group of adjacent network cells that mayinterfere with one another. The member cells of a CSI-RS group and thesize of the group may be semi-statically configured by the networkthrough, for example, RF planning or slowly adapting or dynamicallyadapting the group definitions based upon long term or shorter termobservation of the UEs' RSRP/RSRQ/CQI feedback, UEs' distribution,and/or loading condition, etc.

FIG. 5 is an illustration of an example network mapping showing manycells, with a subset of the cells being arranged in a CSI-RS group. InFIG. 5, cells sharing the same shading are members of the same CSI-RSgroup. For example, the cells labeled A1-A12 are members of a firstCSI-RS group, while cells numbered B1-B12 are members of a second group.In this example, a homogeneous network is illustrate where the groupsize is 12 cells.

Different CSI-RS ports in different cells within a CSI-RS group may beconfigured to transmit mutually orthogonal or quasi-orthogonal CSI-RSs.In that case, orthogonality may be achieved by means of TDM and/or FDMof CSI-RS resources. For example, different REs within the same RB, orin different RBs within a subframe and/or in different subframes, may beused for different CSI-RS ports in different cells within the CSI-RSgroup and/or by code division multiplexing (CDM) where the CSI-RSstransmitted by different cells/CSI-RS ports are on the same set of REsbut are modulated by different orthogonal or pseudo-orthogonalsequences; and/or cyclic shift multiplexing (CSM) where the CSI-RSstransmitted by different cells/CSI-RS ports are on the same set of REsbut are cyclically shifted in the time domain by a delay larger than thechannel delay profile; and/or by a combination of these techniques. TDMof CSI-RS REs can be done by transmitting the CSI-RS REs on differentOFDM symbols within a subframe or by transmitting the CSI-RS REs ondifferent subframes. FDM of CSI-RS REs can be done by transmitting theCSI-RS REs on different OFDM sub-carriers within a RB or acrossdifferent RBs.

A fixed number (N_(CSI-RS)) of orthogonal CSI-RS port resources (in timeand/or frequency and/or code domain and/or cyclically shifted domain)may be reserved for each CSI-RS group. The N_(CSI-RS) could besemi-statically configured and changed from time to time. The N_(CSI-RS)may be equal to or larger than the sum of the required CSI-RS portresources for the CSI-RS group. For example, if the group size is 12 andthe number of CSI-RS port resources required per cell is 2 in the caseof 2 transmit antennas in each cell, then the total number of requiredCSI-RS port resources for the group is 24. Each cell is allocated therequired number of CSI-RS port resources within the set of N_(CSI-RS)port resources. The CSI-RS port resources allocated to different cellswithin the group may vary. A group size of 12 is just an example;typical group sizes may be smaller, e.g. 6. In that case, if each cellrequires 4 transmit antenna ports, the total number of required CSI-RSport resources for a group is 24.

FIG. 6 is an illustration showing reservation of available REs in an RBfor CSI-RS ports within a CSI-RS group. As illustrated in FIG. 6, theN_(CSI-RS) port resources reserved for the CSI-RS group is 48 REs (outof the maximum available REs for CSI-RS transmission of 60). In thisexample, each cell requires two CSI-RS ports corresponding to 4 REs inthe subframe where the CSI-RS is transmitted and the CSI-RS group sizeis 12. A total of 48 available REs are reserved for CSI-RS transmissionby cells within the CSI-RS group. For example, CSI-RS port resources #1and #2 shown in FIG. 6 are used by cell A1 in the CSI-RS group (seecells A1-A12 in FIG. 5, for example); CSI-RS port resources #4 and #5shown in FIG. 6 are used by cell A2 in the CSI-RS group (see cellsA1-A12 in FIG. 5, for example), etc.

In addition to the reuse factor introduced for cells within a CSI-RSgroup proposed above, another level of reuse factor may be used acrossadjacent CSI-RS groups. Different adjacent CSI-RS groups may beallocated different and mutually exclusive/orthogonal sets of N_(CSI-RS)orthogonal CSI-RS port resources. In this manner, the CSI-RS collisionand interference between adjacent CSI-RS groups can be minimized. In theexample shown in FIG. 5, a reuse factor of 3 is introduced as shown bythe 3 different shadings used by different CSI-RS groups. In FIG. 5,CSI-RS groups having the same shading may use the same set of N_(CSI-RS)orthogonal CSI-RS port resources. In some cases, different adjacentCSI-RS groups may use different but not fully mutually exclusive sets ofN_(CSI-RS) orthogonal CSI-RS port resources. Alternatively, theorthogonal resources used by different CSI-RS groups may not be fullyorthogonal. In that case, for each orthogonal resource set that could beallocated to a CSI-RS group, there may be a multi-level score for otherorthogonal resource sets such as “no interference”, “less interference”,“full interference”, etc. When allocating the orthogonal resource setsto different CSI-RS groups, multiple levels of re-use factor could applybased on the scores.

FIGS. 7A-7C illustrates 3 CSI-RS groups where different sets of mutuallyexclusive (or orthogonal) CSI-RS port resources reserved for differentCSI-RS groups are provided using TDM. CSI-RS group #1 (e.g., cellsA1-A12 on FIG. 5) accesses CSI-RS port resources that are reserved onsubframe X (see FIG. 7A), while CSI-RS group #2 (e.g., cells B1-B12 onFIG. 5) accesses CSI-RS port resources that are reserved on subframe Y(see FIG. 7B), and CSI-RS group #3 (e.g., cells C1-C12 on FIG. 5)accesses CSI-RS port resources that are reserved on subframe Z (see FIG.7C). Accordingly, each CSI-RS group is assigned a set of CSI RS portresources being provided at different times.

FIG. 8 is an illustration of multiplexing of mutual exclusive sets ofCSI-RS port resources for different CSI-RS groups within a single RB.Referring to FIG. 8, the CSI-RS group size is five cells and each cellrequires resources for two CSI-RS ports which correspond to four REs inthe subframe where the CSI-RS is transmitted. Therefore, each CSI-RSgroup needs to reserve 20 available REs for the CSI-RS. With a total of60 available REs in an RB for CSI-RS transmission, mutually exclusive(or orthogonal) CSI-RS sets for three CSI-RS groups can be supportedwithin an RB.

In general, mutually exclusive or orthogonal sets of CSI-RS portresources for different CSI-RS groups can be achieved through FDM, e.gdifferent RBs within a subframe are used by different CSI-RS groups forCSI-RS transmission; or a combination of TDM and FDM across differentsubframes and RBs respectively; and different REs within an RB; or CDMfashion; or CSM fashion; or a combination of the above.

In some cases, hopping is performed where the CSI-RS resourcecorresponding to a CSI-RS port used by a particular cell hops from oneCSI-RS resource to another over time, e.g., across different subframeswhere the CSI-RS is transmitted. The hopping of CSI-RS resources usedfor a CSI-RS port may be confined within the set of N_(CSI-RS) resourcesreserved for the CSI-RS group. In some cases, all the cells within thesame CSI-RS group use the same hopping sequence so that no collision ofthe used CSI-RS resources occurs. Accordingly, the objective of hoppingis to randomize the inter-group CSI-RS collision and interference.

As described above, if a reuse factor is introduced for adjacent CSI-RSgroups, the hopping sequence used for different groups within the reusecluster can be different because mutually exclusive sets of N_(CSI-RS)CSI-RS resources are reserved for different groups. For CSI-RS groupsthat use the same set of N_(CSI-RS) CSI-RS resources (e.g., CSI-RSgroups sharing the same shading as shown in FIG. 5), the hoppingsequence used by different groups may be different to randomize CSI-RScollision and interference. For CSI-RS groups that use the partiallyidentical N_(CSI-RS) CSI-RS resources (i.e., partial orthogonal), thehopping sequence used by different groups may also be different.

Using the example shown in FIG. 8, where 10 CSI-RS port resources(corresponding to 20 REs) are reserved for each of CSI-RS groups #1, #2and #3, FIGS. 9A and 9B are illustrations of the CSI-RS port resourcesmapping of CSI-RS group #1 over time. For CSI-RS group #1, as shown inFIG. 8, 10 CSI-RS port resources are reserved, i.e. those REs identifiedby 4, 5, 10, 14, 15, 16, 27, 28, 29 and 30 in FIG. 8. At a particularsubframe A, for example, the mapping of CSI-RS ports of each cell (e.g.,cells #1, #2, #3, #4, and #5) within the CSI-RS group #1 to the actualCSI-RS port resources is shown by the shaded boxes in FIG. 9A. At asecond time, however, when the CSI-RS is transmitted (e.g., subframe B)the mapping of CSI-RS ports to the actual CSI-RS port resources changesto that shown by the shaded boxes in FIG. 9B. In FIG. 9B the mapping ofCSI-RS ports of each cell to the actual CSI-RS port resources is shiftedcyclically among the cells within the CSI-RS group #1. For example, inFIG. 9B (e.g. subframe B), cell #1 uses the CSI-RS port resources ofcell #2 in FIG. 9A (e.g. subframe A); cell #2 in FIG. 9B (e.g. subframeB) uses the CSI-RS port resources of cell #3 in FIG. 9A (e.g. subframeA); and so on and so forth. Accordingly, in FIG. 9A cell #1 uses CSI-RSport resources 4 and 5. However, in FIG. 9B, cell #1 uses CSI-RS portresources 10 and 14 and resources 4 and 5 are used by cell #5.

The hopping of the resource mapping is coordinated among cells withinthe CSI-RS group such that different cells use mutually exclusive CSI-RSport resources. The same hopping sequence may be used for all cellswithin a CSI-RS group, with each cell being offset by a different andpredefined offset value that corresponds to a logical ID associated withthe cell. Different cells within a CSI-RS group have different logicalIDs. In one specific implementation, the logical ID is the physical cellID of the cell. Alternatively, the logical ID may be the logical cell IDof the cell. Different CSI-RS groups may have different hoppingsequences that are randomized by the CSI-RS group ID. Note that thehopping of CSI-RS resource mappings above can be generalized to hoppingover time (e.g., in terms of subframes) and/or over frequency (e.g., interms of RBs).

Each cell in the CSI-RS group may be configured to mute the transmissionof PDSCH REs that coincide with the CSI-RS REs transmitted by othercells within the CSI-RS group. This may result in a reduction of thelevel of interference generated to the CSI-RS of other cells within theCSI-RS group. Alternatively, a cell within the CSI-RS group may onlymute the transmission of PDSCH REs that coincide with the CSI-RS REstransmitted by a subset of the cells within the CSI-RS group. In thatcase, the selection of the subset of cells may be based on theinterference measurements observed. Note that the selection could bechanged from time to time. Within the same CSI-RS group, multiple mutingsubsets could be possible for different cells in the CSI-RS group anddifferent RBs. Generally, the subset of cells may include the strongestinterfering neighbor cells.

In the examples shown in FIG. 6 and FIGS. 7A-7C, each CSI-RS grouprequires 24 CSI-RS port resources (i.e., corresponding to 48 REs). Aspreviously described, generally no more than 16, 24, or 32 Rel-8 PDSCHREs should be punctured or muted in order not to severely degrade theRel-8 PDSCH performance. Therefore, in this case, it may be preferableto distribute the CSI-RS port resources reserved for a CSI-RS group overmultiple subframes, for example across 3 subframes. As such, in eachsubframe 16 REs are used for CSI-RS. FIGS. 10A-10C are illustrations ofdifferent or mutually orthogonal CSI-RS port resources reserved fordifferent CSI-RS groups where 8 CSI-RS port resources (i.e. 16 REs) arereserved for each CSI-RS group in each of the three subframes X (FIG.10A), Y (FIG. 10B), and Z (FIG. 10C). Although in the example shown inFIGS. 10A-10C, the same REs locations are reserved for the same CSI-RSgroup over the three subframes, there may be alternative implementationswhere the REs locations reserved for a CSI-RS group are different acrossdifferent subframes.

In the case that the muting of PDSCH REs is performed for REs thatcoincide with CSI-RS of neighbor cells within the CSI-RS group, thereare cells at the boundary of a CSI-RS group that may experience orgenerate interference from/to neighbor cells in another CSI-RS group. Toavoid interference, a cell may also mute the PDSCH REs that coincidewith CSI-RS of neighbor cells in another CSI-RS group. This may lead toa further increase in the number of PDSCH REs that are punctured/mutedwithin a subframe.

Alternatively, to avoid inter-CSI-RS group interference, the CSI-RS portresources across CSI-RS groups may be multiplexed using CDM or CSM. Inthat case, the same set of available REs within an RB/subframe arereserved for different CSI-RS groups. However, in the case of CDM,different orthogonal or pseudo-orthogonal sequences may be used tomodulate the CSI-RS transmitted by cells in different CSI-RS groups. Toensure orthogonality, the REs used for a CSI-RS port may be adjacent toone another. In the case of CSM, different time domain cyclic shiftdelays may be applied to the CSI-RS transmitted by different CSI-RSgroups.

In some cases, the present implementation may be extended in the case ofa network implementation including a deployment of one or more smallcells. As such, the muting of PDSCH REs may correspond to CSI-RS portresources transmitted by both macro cells and small cells (i.e. CSI-RSsubgroups) within the CSI-RS groups.

Alternatively, each cell may maintain a listing of strongest interferingneighbor cells. The listing may be at least partially included as partof the CoMP measurement set of a UE served by these cells. The list canconsist of cells within the same CSI-RS group as the cell of concernand/or cells in different CSI-RS groups. The CoMP measurement set of aUE is the set of neighbor cells for which a UE measures the CSI usingthe CSI-RS transmitted by the corresponding neighbor cells. To reduceinterference to the CSI-RS transmitted by the list of strongestinterfering neighbor cells, the transmission of PDSCH REs by this cellthat coincide with the CSI-RS REs transmitted by the strongestneighboring cells within the list, may be muted.

The list of strongest interfering neighbor cells of a cell can beconstructed semi-statically by the network through, for example, RFplanning or slowly adapting the listing based upon long term observationof UEs' measurement reports or feedback such as RSRP/RSRQ report, CQIreport, etc.

In some cases, the present system may be extended in the case of anetwork implementation including a deployment of one or more smallcells. As such, the interfering neighbor cells of a cell (either macrocell or small cell) include both interfering macro cells as well asoverlaid small cells.

When implementing the present system, to decode received PDSCHtransmissions, and to detect the CSI-RSs transmitted by the UE's servingcell and neighbor cells in the UE's CoMP measurement set, a UE may needto have information related to the CSI-RS ports transmitted by the UE'sserving cell, the CSI-RS ports transmitted by neighbor cells in the UE'sCoMP measurement sets and the PDSCH REs that are muted. There areseveral mechanisms or processes that a eNB and/or UE may implement inorder for the UE to determine the necessary information.

First, the physical location of REs available for potential CSI-RStransmission within an RB may be predefined in a specification orbroadcast in a system information block (SIB). In the example shown inFIG. 6, there are a total of 60 REs available and their locations may bepredefined or communicated to one or more UEs using a system informationblock (SIB).

Each available RE (and possibly CDM sequence or CSM cyclic shift delay)is indexed by a number to associate the available RE and/or CDM sequenceand/or CSM cyclic shift delay with the CSI-RS port resource. Thenumbering may be predefined in a specification or broadcast in an SIB.In the example shown in FIG. 6, there are 30 numbered CSI-RS portresources. Each CSI-RS port resource in the example corresponds to twoREs. Each CSI-RS port resource can be used for CSI-RS transmission for aCSI-RS antenna port. A larger number of CSI-RS port resources can bedefined if full orthogonality between CSI-RS port resources (e.g. inFDM, TDM, CDM or CSM domain) does not need to be maintained.

The CSI-RS port resources (and their corresponding subframes and RBswithin those subframes) reserved for a CSI-RS group and the CSI-RS groupID may be signaled by each cell within the CSI-RS group to the UEsserved by the cell. The signaling may be broadcast through an SIB orsent via dedicated signaling to each UE. In the example shown in FIG. 8,the CSI-RS port resources reserved for CSI-RS group #1 are indexed by 4,5, 10, 14, 15, 16, 27, 28, 29 and 30. In addition, the subframe numbers(within a radio frame) and RBs within those subframes where thecorresponding CSI-RS port resources are reserved may also be signaled tothe UEs. If CSI-RS port resources hopping is employed within the CSI-RSgroup, a set of reserved CSI-RS port resources for the CSI-RS group willchange from one subframe and/or RB to another. The hopping sequencebased on subframe number and/or RB number can be predefined in thespecification.

The CSI-RS port resources reserved for a CSI-RS group may be mapped tocertain logical CSI-RS port resource indices as shown in FIG. 11. Thesignaling of the mapping may be implicit such that the CSI-RS portresources assigned for the CSI-RS group are ordered according to theirlogical indices in the broadcast or dedicated signaling message (e.g. aRadio Resource Control (RRC) message). Alternatively, the logicalindices may be assigned implicitly based upon incremental values of theassigned CSI-RS port resource indices. Alternatively, the mapping ofCSI-RS port resources to logical CSI-RS port resource indices areexplicitly indicated in the broadcast or dedicated signaling sent by aserving cell to the cell's UEs.

The logical CSI-RS port resource indices may be used for mapping CSI-RSports of each cell to the actual REs used for the CSI-RS transmission.For example, as shown in FIG. 11, logical CSI-RS port resources #1 and#2 are assigned to cell A1, logical CSI-RS port resources #3 and #4 areassigned to cell A2 and so on. The mapping of CSI-RS ports of a cell tothe logical CSI-RS port resources can be based on the logical IDassigned to a cell within the CSI-RS group based on a predefined mappingrule defined, for example, in a specification. In one implementation,the logical ID is the same as the physical cell ID (PCI).

If CSI-RS hopping is enabled, the mapping of CSI-RS ports of a cell tothe logical CSI-RS port resources can be based on the logical IDassigned to a cell within the CSI-RS group and the subframe and/or theRB on which the CSI-RS is transmitted, based on a predefined mappingrule defined, for example, in a specification. The same hopping sequencefor CSI-RS ports to logical CSI-RS port resources mapping may be usedfor all the cells within the CSI-RS group, with each cell being offsetby a different and predefined offset value that corresponds to a logicalID associated with each cell. In one implementation, the hoppingsequence associated with a CSI-RS group may be defined based on theCSI-RS group ID.

A UE may be signaled by the UE's serving cell (via broadcast ordedicated signaling such as RRC signaling) with the logical IDassociated with the UE's serving cell and the number of CSI-RS portsthat the UE's serving cell transmits (which corresponds to the number oftransmit antennas of the UE's serving cell). Based upon the logical IDand the information described above, the UE can derive the CSI-RS portresources used for CSI-RS transmission by the UE's serving cell. In oneimplementation, the logical ID is the same as the physical cell ID(PCI). In this case, the UE may derive the PCI from the synchronizationchannel, for example.

A UE in CoMP operation may also be signaled by the UE's serving cell(via broadcast or dedicated signaling such as RRC signaling) with thefollowing information of each neighbor cell in the UE's CoMP measurementset. If the neighbor cell is in a different CSI-RS group (i.e., aneighbor CSI-RS group), and a reuse factor is introduced across CSI-RSgroups (as discussed above), the following information of the neighborCSI-RS group may be signaled: 1) CSI-RS port resources (and theircorresponding subframes and RBs within those subframes) reserved for theneighbor CSI-RS group; 2) the mapping of the reserved CSI-RS portresources to logical CSI-RS port resource indices; and 3) CSI-RS groupID. Additional information to be signaled may include a logical IDassociated the neighbor cell, and the number of CSI-RS ports that theneighbor cell transmits or specific CSI-RS ports of the neighbor cellthat the UE should measure the CSI.

Based upon this information, the UE can derive the CSI-RS port resourcesused for CSI-RS transmission by each neighbor cell and therefore measureand report the CSI of specific CSI-RS ports in each neighbor cell in theUE's CoMP measurement set.

A UE may also be signaled with the logical IDs or PCI and number ofCSI-RS ports (or specific CSI-RS ports) associated with a list ofneighbor cells within the same CSI-RS group as the UE's serving cell sothat the UE can derive which PDSCH REs transmitted by the UE's servingcell are muted based on the CSI-RS port resources transmitted by thelist of neighbor cells. In addition, a UE may also be signaled thefollowing information of a list of neighbor cells in a different (orneighbor) CSI-RS group in order to derive which PDSCH REs transmitted bythe UE's serving cell are muted based on the CSI-RS port resourcestransmitted by this list of neighbor cells: If reuse factor isintroduced across CSI-RS groups (as discussed above), the followinginformation of the neighbor CSI-RS group may be signaled: 1) CSI-RS portresources (and their corresponding subframes and RBs within thosesubframes) reserved for the neighbor CSI-RS group; 2) the mapping of thereserved CSI-RS port resources to logical CSI-RS port resource indices;and 3) CSI-RS group ID. Additional information may include a logical IDassociated with the neighbor cell, and the number of CSI-RS ports (orspecific CSI-RS ports) the neighbor cell transmits.

The following procedures allow a UE to acquire information related toCSI-RS and number of antenna ports of its serving cell. Similar to LTERel-8, an LTE-A UE in Idle mode decodes the Physical Broadcast Channel(PBCH) to read the Master Information Block (MIB) of the UE's(re)selected cell. The UE obtains the antenna configuration of the cellused for transmitting CRS, common control channels (e.g. PDCCH, PCFICH,PHICH etc.) and PDSCH carrying SIBs through blind decoding and CRCde-masking of the PBCH using hypothesis of 1tx, 2tx or 4txconfigurations. While entering RRC_Connected mode, or while inRRC_Connected mode, the UE may acquire information related to the CSI-RSof the UE's serving cell as previously described through the decoding ofSIB carried in the PDSCH. The CSI-RS information of the serving cell canbe included in a new SIB introduced for LTE-A or in one or more newinformation elements (lEs) introduced in existing SIBs. A UE inRRC_Connected mode may be further signaled by the UE's serving cell thenumber of CSI-RS ports (or specific CSI-RS ports) and the neighbor cellsfor which the UE should measure/report the CSI and CQI. This may beassociated with the transmission mode configured for the UE.

In some cases, the CSI-RS is only transmitted in an MBSFN subframe or ina subset of the MBSFN subframes. One or more MBSFN subframes can bedefined within a radio frame where the CSI-RS is transmitted.Alternatively, the CSI-RS is only transmitted in an LTE-A subframe,which is a subframe that only supports LTE-A UEs. The previouslydescribed concepts of CSI-RS port resources, CSI-RS groups, reuse factoracross CSI-RS groups, and muting of PDSCH REs may be applied in thisimplementation.

Because legacy Rel-8 UEs only decode the first two symbols of an MBSFNsubframe for PDCCH information, the remaining symbols in an MBSFNsubframe may not need to transmit the Rel-8 CRS. Accordingly, the numberof available REs within the MBSFN subframe becomes larger. FIG. 12 is anillustration of available REs in an RB for CSI-RS in a MBSFN subframe.As shown in FIG. 12, the number of available REs for CSI-RS is 120corresponding to 60 CSI-RS port resources in the case where each CSI-RSport resource corresponds to two REs. As the overhead of CRS is reduced,i.e. 16 CRS REs are no longer needed in an MBSFN subframe, some of theavailable RE resources can be used for CSI-RS purposes, i.e. either forCSI-RS transmission or muting of PDSCH REs.

One or more specific MBSFN subframe within a radio frame or withinmultiple radio frames may be used for CSI-RS transmission only withoutPDSCH transmission. The special MBSFN subframe may be used by all cellsin the CSI-RS group or all cells in the network for CSI-RStransmissions.

In some cases, the CSI-RS group, reuse factor across CSI-RS groups,hopping of CSI-RS, and PDSCH REs muting concepts are only applied to asubset of CSI-RS ports transmitted by each cell. For example, if eachcell transmits a total of 8 CSI-RS ports, only N (where N<8) of theCSI-RS ports may be implemented in accordance with the conceptsdescribed above. For example, only N CSI-RS port resources used by eachcell may be orthogonal to those used by neighbor cells within the CSI-RSgroup. The reuse factor can be introduced across CSI-RS group and thehopping of CSI-RS port resources may be applied to only N CSI-RS portsin each cell. The muting of PDSCH REs of a cell may only be applied tothose REs that coincide with the CSI-RS port resources of the N CSI-RSports of neighbor cells. In some cases, the subset of N CSI-RS ports arethose that are used for CoMP purposes. The remaining (8-N) CSI-RS portsof each cell may occupy CSI-RS port resources that are not orthogonal toeach other or partially orthogonal to each other.

In some cases, the muting of specific PDSCH REs is applied to all RBstransmitted by a cell either based on CSI-RS grouping or based on astrongest interfering neighbor cell list. However, because mutingdegrades the PDSCH performance of legacy UEs, it may be better that notall RBs within the system bandwidth are affected by muting.

For UEs that are closer to the cell center where CoMP is not applied,there may be no need for those UEs to measure the neighbor cells'CSI-RS. Therefore, muting of PDSCH REs may not provide any benefit tothose UEs. On the other hand, for UEs that are closer to the cell edge,the list of strongly interfering neighbor cells may be different fordifferent UEs' location. FIG. 13 is an illustration of a CSI-RS groupingwithin a network showing the strongest neighbor cells to UEs atdifferent location within the cell. Referring to FIG. 13, within cellA1, a first UE is located at the location marked by ‘X’ while a secondUE is located at the location marked by ‘Y’. It can be seen that for theUE located at ‘X’, the likely strong interfering neighbor cells arecells A12, A4 and A5. For the second UE located at ‘Y’, the likelystrong interfering neighbor cells are cells A6, A7, A2, and A8. Althoughthe CSI-RS group size may still be 12 as shown in this example where thecells within the group transmit mutually orthogonal CSI-RSs as discussedabove, the muting does not need to occur for PDSCH REs that coincidewith the REs of CSI-RS transmitted by all the 12 cells within the group.The muting of PDSCH REs may only be necessary for those REs thatcoincide with the CSI-RS transmitted by the strong neighbor cells andwithin those RBs that are used by the UEs of concern to measure theCSI-RS of the strong neighbor cells.

To avoid unnecessary muting, the RBs transmitted by a cell within thesystem bandwidth may be divided into different RB groups. A particularRB group may be identified by a base station and the identity of the RBgroup may be transmitted to a UE. For the CSI-RS groups shown in FIG.13, for example, the available RBs may be partitioned into three RBgroups as shown in FIG. 14. FIG. 14 is an illustration of RBs based onPDSCH REs muting requirements. The RBs within each RB group can becontiguous or non-contiguous. In this example, the first group of RBs300 is used for PDSCH transmission to cell center UEs that do notrequire CoMP. Because cell-center UEs do not need to measure the CSI-RSof neighbor cells, no muting of PDSCH REs is required for the firstgroup of RBs 300. The first group of RBs 300 can also be used for PDSCHassignment to legacy Rel-8 UEs because the impact caused by CSI-RStransmission will be reduced.

The second group of RBs 302 is used for PDSCH transmission to cell edgeUEs that require CoMP and are located at the cell edge region such thatcells A4, A5, A12 are the strong interfering neighbor cells (e.g., fortransmissions to UEs at location X). In that case, muting is done on thePDSCH REs that coincide with the CSI-RS transmitted by those neighborcells.

The third group of RBs 304 is used for PDSCH transmission to cell edgeUEs that require CoMP and are located at the cell edge region such thatcells A2, A6, A7, A8 are the strong interfering neighbor cells (e.g.,for transmissions to UEs at location Y). In that case, muting is done onthe PDSCH REs that coincide with the CSI-RS transmitted by theseneighbor cells.

The implementation illustrated in FIG. 14 may be generalized to definedifferent numbers of RB groups within a cell where each group has aunique set of PDSCH REs that are muted to avoid interference caused tothe CSI-RS transmitted by a set of strong interfering neighbor cells. Inthat case, each RB group may be targeted for specific groups of UEs thatobserve a specific set of strong interfering neighbor cells. The RBgroup described above can also be applied to the time domain or timedomain plus frequency domain where different RB groups can be definedacross different subframes with different periods of occurrence.Different CSI-RS power boosting levels can be applied to different RBgroups to improve the serving and neighbor cells' CSI-RS detectionreliability. For example, the first RB group used to serve cell centerUEs may not require CSI-RS power boosting, i.e. the power boosting levelis set to 0 dB. The second and third RB groups (e.g., groups 302 and306) which are used to serve cell edge UEs may be configured with thesame or different power boosting levels greater than 0 dB.

To reduce complexity and to reduce the impact to scheduling efficiency,the number of RB groups may be kept relatively small. In one example,both UE1 and UE2 are served by cell A1. UE1 has cell A2 and cell A3 asstrong interfering neighbor cells while UE2 only has cell A2 as a stronginterfering neighbor cell. Although UE1 and UE2 may be grouped intoseparate RB groups, to reduce the number of different RB groups, UE1 andUE2 can be grouped into the same RB group which defines neighbor cell A2and cell A3 as strong interfering neighbor cells for both UE1 and UE2.In that case, the PDSCH REs within each RB of the RB groups thatcoincide with the CSI-RS transmitted by cell A2 and cell A3 may bemuted. Although this incurs unnecessary muting overhead for UE2, itreduces the number of RB groups that need to be defined for a particularcell, thus reducing the impact on scheduling efficiency. Alternatively,for simplicity, the number of RB groups may be set to only 2, with afirst group being reserved for non-CoMP UEs and a second group beingreserved for CoMP operation. Note that different cells or subset ofcells may have different configurations.

In this implementation, a UE may be configured to report CQI and CSI ofone or more assigned RB groups. The CQI and CSI reporting configured fora UE on each assigned RB group can be the average CQI/CSI across all theRBs in the RB group and/or the CQI/CSI of certain preferred sub-bands(where each sub-band consists of a number of adjacent RBs) within the RBgroup. A UE may also be configured to report the wideband and/orsub-band CQI/CSI of one or more preferred RB groups among the assignedRB groups or report the preferred sub-bands among all of the assigned RBgroups.

In some cases, the system may be extended in the case of a networkimplementation including a deployment of one or more small cells. Assuch, the set of strong interfering neighbor cells that define the PDSCHREs muting within an RB group may include both neighboring macro cellsas well as overlaid small cells.

Because the muting or non-muting of specific PDSCH REs transmitted froma cell may affect the level of interference caused to the CSI-RSs ofneighbor cells, the RB grouping may be coordinated among neighboringcells such that the same group of RBs are used to serve the set of UEsin neighboring cells that are observing the same set of stronginterfering cells plus their serving cell. Using the example of three RBgroups illustrated in FIG. 14, RB group 300 may be used to serve cellcenter UEs that do not use CoMP. This same group of RBs can be used byeach neighboring cell to serve its own cell center UEs in the samemanner. Accordingly, even though there is no muting of PDSCH REs inthose RBs, there is no impact on the CSI-RS detection of these cellcenter UEs in the various neighboring cells. As an example, for RB group302, cells A1, A4, A5, and A12 of FIG. 13 may be defined as mutuallyinterfering cells associated with RB group 302. RBs defined for RB group302 may be used by each of the mutually interfering cells to serve theirUEs that have a list of serving cell plus strong interfering cells thatincludes cells A1, A4, A5, and A12 of FIG. 13. Each mutually interferingcell may perform PDSCH RE muting for REs that coincide with the CSI-RSof the other mutually interfering cells. Similarly, for RB group 304,the associated mutually interfering cells are A1, A2, A6, A7, and A8.RBs defined for RB group 304 may be used by each of the mutuallyinterfering cells to serve the cell's UEs that have a list of servingcell plus strong interfering cells that includes cells A1, A2, A6, A7,and A8.

In accordance with the present implementation, one or more RB group isdefined for each cell. The set of RBs reserved for a first RB group maybe mutually exclusive from the set of RBs reserved for another RB group.Each RB group has an associated list of mutually interfering cells. Thelist of mutually interfering cells associated with an RB group may becalled the CSI-RS muting group. Each of the mutually interfering cellswithin the CSI-RS muting group uses the RBs reserved for thecorresponding RB group to serve its own UEs that observe stronginterference from the cells within the CSI-RS muting group excluding theUE's serving cell. Each of the cells within the CSI-RS muting group maythen perform PDSCH RE muting on REs that coincide with the CSI-RStransmitted from other cells within the CSI-RS muting group. Each of thecells within the CSI-RS muting group may be configured to apply acertain pre-configured power boosting level to the CSI-RS transmission.The power boosting level may be set to be the same among all the cellsor different for different cells.

To reduce the number of RB groups that need to be defined for a cell inorder to reduce the impact on scheduling efficiency, UEs that observedifferent strong interfering neighbor cells can be grouped together andserved by the same RB group. For example, an RB group may be associatedwith a first, second and third cell as mutually interfering cells. UE1and UE2 are served by the first cell. UE1 observes the second and thirdcells as strong interfering cells and therefore is served by this RBgroup. UE2 observes only the second cell as a strong interfering cell.In this example, however, UE2 can also be served by this RB group. Thisintroduces unnecessary muting of PDSCH REs for UE2, but avoids addinganother RB group to define the first and second cell as mutuallyinterfering cells.

In some cases, the present system may be extended in the case of anetwork implementation including a deployment of one or more smallcells. As such, the CSI-RS muting group may consist of mutuallyinterfering macro cells as well as overlaid small cells within themutually interfering macro cells. To reduce the number of PDSCH REs thatneeds to be muted, small cells that are located in the coverage area ofdifferent macro cells within the CSI-RS muting group can be assigned thesame CSI-RS port resources.

A cell or base station can identify a listing of strong interferingcells observed by an UE using the UE's measurement report (e.g., an RSRPor RSRQ report) or a combination of measurement reports received fromother UEs. Based upon the RSRP/RSRQ report from the UEs served by thecell, and through coordination with neighbor cells, a cell can determinethe number of RB groups to be constructed, the interfering neighborcells associated with each RB group, and the number of RBs assigned toeach RB group. The configuration may be updated from time to time. Insome cases, a cell coordinates with the cell's neighboring cells todetermine the RB grouping. The grouping may also depend upon the numberof users involved in the CoMP set, traffic loading situations, orneighboring cell loading conditions, etc.

Based upon the RSRP/RSRQ report received from one or more UE, the celldetermines the RB group to which the UE should be assigned. For example,a UE may be assigned to an RB group where the UE's CoMP measurement setis a subset of the interfering neighbor cells associated with the RBgroup. Alternatively, a UE can be assigned multiple RB groups to allowbetter resource multiplexing among UEs served by the cell and overallscheduling efficiency.

In addition to the use of signaling to indicate the CSI-RS transmittedby the serving cell, the CSI-RS transmitted by neighbor cells in theCoMP measurement set and the muted PDSCH REs to the UE, additionalsignaling may be used to assign one or more RB groups to the UE inaddition to corresponding PDSCH REs muting of an assigned RB group. Forexample, the following information associated with an RB group may besignaled to the UE (e.g., using an SIB broadcast or dedicated RRCsignaling). The set of RBs belonging to an RB group—the set may becontiguous, non-contiguous, or a combination of both. The PDSCH REswithin the set of RBs that are muted. The UE may be signaled with thelogical IDs and number of CSI-RS ports (or specific CSI-RS ports)associated with a list of neighbor cells (called interfering cell group)within the CSI-RS muting group associated with the RB group so that theUE can derive which PDSCH REs transmitted by its serving cell within theRB group are muted based on the CSI-RS port resources transmitted bythis list of neighbor cells. In one implementation, the UE's CoMPmeasurement set is a subset of the above-mentioned list of neighborcells. Finally, the UE may be signaled with the power boosting level forCSI-RSs transmitted by cells in the CSI-RS muting group associated withthe RB group.

If the above information related to an RB group is broadcast to the UE,the UE may be separately assigned or de-assigned an RB group viadedicated signaling (e.g. dedicated RRC signaling). Alternatively,dedicated signaling (e.g. dedicated RRC signaling) can be used toassign/de-assign an RB group to or from the UE and at the same timeprovide the above information associated with an assigned RB group tothe UE.

In the case of a heterogeneous network, small cells may be locatedwithin the coverage areas of macro cells. In that case, the CSI-RStransmitted by a small cell may be orthogonal to the CSI-RS transmittedby the macro cell within which the small cell is located as well as theCSI-RS transmitted by other interfering macro and small cells.

Due to the low transmit power of small cells, the coverage of smallcells may not overlap. FIG. 15 is an illustration of an example networkincluding several macro cells with small cells #1, #2, and #3 disposedwithin the macro cells. As shown in FIG. 15 macro cells A1 and A4 mayinterfere directly with small cells #1, #2, and #3. Additionally,surrounding macro cells may also interference with small cells #1, #2,and #3. However, because the small cells do not generally interfere withone another, the same CSI-RS port resources can be transmitted bynon-overlapping small cells.

In one implementation, the CSI-RS group concept introduced above isextended so that each of the small cells may be added as an independentmember of the CSI-RS group. For example, the CSI-RS group shown in thisexample is extended from a group size of 12 to 13, with small cellsSC#1, SC#2, SC#3 each using the same CSI-RS port resources andcorresponding functionally to a CSI-RS group member A13. As shown inFIG. 15, because the small cells do not overlap, small cells #1, #2, and#3 can each use the same CSI-RS ports defined for CSI-RS group membercell A13, whether or not the small cells are within the same macro cellcoverage or different macro cell coverage. In this example, the group ofsmall cells that use the same CSI-RS port resources are defined withinthe CSI-RS group as a CSI-RS subgroup.

When a small cell is installed or powered on, the small cell may beconfigured to detect the interference environment, i.e. interferingneighbor cells and report those interfering cells to the self organizingnetwork (SON) manager. The SON manager may then assign the same CSI-RSport resources to non-overlapping small cells. For example, withreference to FIG. 15, the SON may receive reports from small cells #1and #2 that macro cell Al is interfering. In that case, neither smallcell #1 or #2 reports that the other small cell is interfering.Accordingly, small cells #1 and #2 do not overlap with one another andmay be assigned the same CSI-RS resources.

FIG. 16 is an illustration of an alternative small cell networkdeployment where one or more of the small cells overlap. In FIG. 16, thecoverage of SC#3 and SC#5 overlaps, and the coverage of SC#2 and SC#4overlaps. Due to the interference between the overlapping small cells,the CSI-RS transmitted by these overlapping small cells is orthogonal.As a result, the CSI-RS group size becomes 14. The macro cells (A1-A12)provide 12 members of the CSI-RS group. Small cell #1, #2, and #3 eachprovide a single member as they do not interfere with one another. Smallcells #4 and #5 each provide a single additional member of the CSI-RSgroup as they are each allocated CSI-RSs that are orthogonal to thoseused by small cells #1, #2 and #3.

In the example shown in FIG. 16, two CSI-RS subgroups are defined withinthe CSI-RS group. CSI-RS subgroup 1 consists of SC#1, SC#2 and SC#3which transmit CSI-RS port resources that correspond to A13. CSI-RSsubgroup 2 consists of SC#4 and SC#5 which transmits CSI-RS portresources that correspond to A14. Based upon reported interferencesituations, the network may select the CSI-RS to be assigned to thesmall cells. When a small cell is installed or powered on, for example,the small cell may detect the interference environment, i.e. interferingneighbor cells and report the interference environment to the selforganizing network (SON) manager. The SON manager may then assigndifferent CSI-RS port resources to the overlapping small cells.

Alternatively, the CSI-RS port resources used by some of the small cellsdo not have to be orthogonal to all the CSI-RS port resources used bythe macro cells within the CSI-RS group. Depending upon the location ofa small cell, for example, the CSI-RS port resources used may only needto be orthogonal to the CSI-RS port resources used by interfering macrocells (and other small cells with overlapped coverage area) within theCSI-RS group.

For example, FIG. 17 is an illustration of a network implementationincluding an overlay of small cells on top of macro cell coverage, insome cases the coverage of the small cells overlaps. As shown in FIG.17, the interfering macro cells to small cell #2 and small cell #4 arethe cells corresponding to A1, A3, A4, A5, and A12 within the CSI-RSgroup. Therefore, small cell #2 and small cell #4 can use the CSI-RSport resources corresponding to A2, A6, A7, A8, A9, A10, A11 as long assmall cell #2 and small cell #4 use different CSI-RS port resourcesbetween themselves. In this example, small cell #1 and small cell #3 areclose to the cell site of Al and observe minimal interference from otherneighbor macro cells. Therefore, small cell #1 and small cell #3 can useany of the CSI-RS port resources corresponding to A2 through A12 as longas small cell #1 and small cell #3 use different CSI-RS port resourcesbetween themselves. As such, the CSI-RS port resources used by smallcell #1 and small cell #3 may be the same as those used by small cell #2and small cell #4 as the cells do not have overlapping coverage area.Alternatively, the CSI-RS port resources of the macro cells within theCSI-RS group may be re-used in the small cells.

In the case that the number of small cells is relatively large, twolayers of CSI-RS allocation may be used. The small cells may beallocated to a first tier grouping and the macro cells may be allocatedto a second tier. The common CSI-RS group may be assigned to cells ofboth the first and second tiers, but any first tier group having thesame CSI-RS resource allocation may not be overlapped in coverage areawith any second tier group having the same CSI-RS resource allocation.In some cases, overlapping may be allowed (but limited) if theinterference scenario is controllable.

In the case where a small cell moves from one location to another, e.g.,in the case of a mobile relay node or moving pico cell, a separate setof CSI-RS port resources may be reserved for moving small cells. TheseCSI-RS port resources may be separate or orthogonal from those used formacro cells and/or static small cells. Accordingly, as a small cellmoves from one location to another, the small cell's CSI-RS will notinterfere with the CSI-RS transmitted by other macro cells or staticsmall cells.

Different moving small cells may be assigned different CSI-RS portresources within the set of CSI-RS port resources reserved for movingsmall cells. To avoid CSI-RS interference between moving small cells,moving small cells located within the same macro cell coverage area mayuse different CSI-RS port resources. As a moving small cell moves fromone macro cell coverage area to another, the CSI-RS port resources usedby the moving small cell may change. The allocation, reservation andcoordination of CSI-RS port resources for moving small cells, staticsmall cells and macro cells may be performed by a SON manager.

Alternatively, when the small cell is moving, the moving small cell isconfigured to continuously monitor strongly interfering neighboringcells. In that case, a CSI-RS is selected and re-selected based upon theupdated strong interfering neighboring cell set captured and broadcastby the moving small cell to reduce the interference. This can be done bythe network in a distributed manner or centralized control manner, e.g.by an SON manager. The small moving cell may send the updated CSI-RS toattached UEs via BCCH signaling or dedicated signaling.

Alternatively, the set of CSI-RS port resources used by the moving smallcells may not be fully separated from those used by macro cells and/orstatic small cells. The CSI-RS port resources used by a moving smallcell may be based on the current location of the moving small cell andthe interfering neighbor macro cells. The CSI-RS port resources used bya moving small cell may be orthogonal to those used by interfering macrocells as well as those used by other small cells (moving or static)located within the coverage of the interfering macro cells.Alternatively, the CSI-RS port resources used by a moving small cell areorthogonal to those used by interfering macro cells as well as thoseused by other small cells (moving or static) located within the samemacro cell coverage area as the moving small cell. The CSI-RS portresources used by a moving small cell may be orthogonal to those used bythe macro cell where the moving small cell is currently located as wellas those used by other small cells (moving or static) located within thesame macro cell coverage area as the moving small cell. As the movingsmall cell moves, the CSI-RS port resources used may change based on theinterference environment.

In the case of UEs moving at a high rate of speed, a more frequentoccurrence of CSI-RS subframe broadcasting may be used to providereliable channel information for efficient scheduling, precoderselection and link adaptation. For example, in one implementation of thepresent system, the CSI-RS is transmitted as described above, but anadditional occurrence of CSI-RS broadcast is performed within the radioframe and is intended for higher speed mobiles. The additional set ofCSI-RSs may be referred to as a supplemental CSI-RS.

The periodicity of the supplemental CSI-RS subframe may be the same orgreater than the normal CSI-RS subframe periodicity. For example, theperiodicity of both the normal CSI-RS subframe and the supplementalCSI-RS subframe are the same, however the location of the subframes areinterleaved ensuring maximum separation in time. FIG. 18 is anillustration of interleaved normal and supplemental CSI-RS subframelocations, each having a periodicity of 10 subframes (or one frame).

The transmission of the supplemental CSI-RS may be semi-staticallyconfigured by the network. The configuration of the supplemental CSI-RStransmission may be changed from time to time based on current radioconditions, mobile speed, and cell loading conditions. For example, inthe case that a cell is over-loaded, the supplemental CSI-RStransmission may be stopped to allow more user data transmission.Alternatively, when cell loading is light and the number of high speedUEs is large, the network may configure more supplemental CSI-RStransmissions to allow more accurate CSI estimation, for example, alonghigh-speed roadways, railways, or other high-speed avenues where UEs arelikely to be moving at high speed.

The normal CSI-RS and supplemental CSI-RS broadcasts may occur on thesame subframe for certain subframes. In that case, normal CSI-RSs andsupplemental CSI-RSs may be transmitted on different RBs or the sameRBs.

The number of antenna ports supported by the supplemental CSI-RS may beequal to or less than that of the normal CSI-RS to limit overhead.Supplemental CSI-RS antenna ports may be mapped to RE's in the samepatterns as those presented for the normal CSI-RS. For example, thenormal CSI-RS may use N (e.g. N=8) antenna ports per RB, however thesupplemental CSI-RS may only support M≦N (e.g. M=2) antenna ports perRB.

In some cases, the number of antenna ports and RE's used for thesupplemental CSI-RS is smaller than the normal CSI-RS as a lower thenumber of antennas or virtual antenna streams is supported. For example,if the normal CSI-RS supports 8 antenna ports with 1 RE per RB for eachantenna port, the supplemental CSI-RS may support 2 antenna ports with 1RE per RB for each antenna port.

In some configurations, UEs can use the normal CSI-RS for CSI estimationof a larger set of antennas and can use the supplemental CSI-RS foradditional CSI estimation (i.e. more frequent information) for a subsetof the antennas. Some UEs may use only those antennas or spatialdimensions that are common to both normal and supplemental CSI-RS forCSI estimation. This can benefit higher speed UEs that may require fewerantennas and in general lower rank transmissions; however thisconfiguration may require more frequent CSI-RS broadcasts due to fasterchanging channel conditions.

A UE that has slowly changing channel conditions and is capable ofsupporting higher rank transmission may ignore the supplemental CSI-RSif the UE does not have sufficient information (e.g., how eachsupplemental CSI-RS antenna port is mapped to or linearly/non-linearlycombined from the normal CSI-RS antenna ports) to resolve individualnormal CSI-RS antenna ports from the supplemental CSI-RS antenna ports.In general, however, if the UE has the supplemental CSI-RS information,for example, via the reception of the BCCH or dedicated RRC signaling,the UE may use the CSI-RS for better CSI measurement.

UEs can select, or alternatively can be configured by the network, touse and report either the normal CSI-RS, the supplemental CSI-RS or acombination of normal and supplemental CSI-RSs for CSI estimation.Likewise, the feedback on the UL from the UE may indicate whether theCSI feedback is based on the format of the supplemental CSI-RS ports, orthe normal CSI-RS ports. Alternatively, UEs may be configured by thenetwork to provide CSI feedback according to one of the CSI-RS formats.

A UE using normal CSI-RS ports may also feedback CSI according to theformat of the supplemental CSI-RS antenna ports depending on the mappingrule between the normal CSI-RS antenna ports and the supplemental CSI-RSantenna ports and whether the UE provided sufficient informationregarding the mapping rule.

In one implementation, the antenna ports for the supplemental CSI-RS area subset of those used for the normal CSI-RS and may be mapped accordingto a pre-determined allocation of ports, for example. An example mappingof ports is illustrated below in Table 2. The system can then use one ofthese configurations that may be indicated to a UE by the row index, forexample. To limit the signaling needed for the row index, differenttables can be used for each number of normal CSI-RS antenna ports.Alternatively, the network may signal the supplemental CSI-RS port andthe mapped normal CSI-RS port in a list-based format via RRC signaling.In some cases the table may indicate both the antenna port mapping andthe number of supplemental CSI-RS ports, given a specific number ofnormal CSI-RS antenna ports. The table could be semi-staticallyconfigured by the base station.

Accordingly, Table 2 and Table 3 illustrate possible mapping tables for4 and 8 normal CSI-RS antenna ports, respectively. The number ofsupplemental antenna ports as well as the mapping rule may then beindicated by specifying a row index for the table.

TABLE 2 Supplemental Row CSI-RS 1 2 3 4 Index Port Corresponding NormalCSI-RS Port 1 1 2 3 4 2 1 3 — — 3 2 4 — — 4 1 — — —

TABLE 3 Supplemental CSI-RS 1 2 3 4 5 6 7 8 Row Index Port CorrespondingNormal CSI-RS Port(s) 1 1 2 3 4 5 6 7 8 2 1 3 5 7 — — — — 3 2 4 6 8 — —— — 4 1 4 — — — — — — 5 5 8 — — — — — — 6 1 8 — — — — — — 7 1 2 — — — —— — 8 1 — — — — — — —

In some cases, the number of row indices for each table (where there isa table for each number of normal CSI-RS antenna ports) may be the same.This may allow the field size for the indication of the row index forsupplemental CSI-RS ports and port mapping to be constant regardless ofthe number of normal CSI-RS antenna ports.

UEs that use both normal and supplemental CSI-RS may have more frequentCSI on the antenna ports that are contained in the normal and supplementantenna port sets, than those UEs that use normal CSI-RS only. UEs usingsupplemental CSI-RS ports may be configured to feedback CSI accordinglyto the format of the supplemental CSI-RS antenna ports.

In some cases, the antennas ports for the supplemental CSI-RS are linearor non-linear combinations of those used for the normal CSI-RS. Inpractice, a table or precoding matrices set may be created for selectedmappings of ports. The system can then use one of these configurationswhich can be indicated by the row index or precoding matrix index (PMI).To limit the signaling needed for the row index or PMI, different tablesor different sets of precoding matrices can be used for each number ofnormal CSI-RS antenna ports.

In some cases, the table or precoding matrices set may indicate both theantenna port mapping and number of supplemental CSI-RS ports, given aspecific number normal CSI-RS antenna ports. Table 4 and Table 5illustrate example possible mapping tables for 4 and 8 normal CSI-RSantenna ports mapping to supplemental CSI-RS antenna ports. In theexample, linear or other combinations of the normal CSI-RS antenna portsmay be used to form the supplemental CSI-RS antenna ports. In thesecases, the number of supplemental CSI-RS antenna ports as well as themapping rule may be indicated by specifying a row index for the table

TABLE 4 Supplemental Row CSI-RS 1 2 3 4 Index Port Corresponding NormalCSI-RS Port 1 1 2 3 4 2 1 + 2 3 + 4 — — 3 1 + 2 4 — — 4 1 — — —

TABLE 5 Supplemental CSI-RS 1 2 3 4 5 6 7 8 Row Index Port CorrespondingNormal CSI-RS Port(s) 1 1 2 3 4 5 6 7 8 2 1 + 2 3 + 4 5 + 6 7 + 8 — — —— 3 1 + 5 2 + 6 3 + 7 4 + 8 — — — — 4 1 + 2 + 3 + 4 5 + 6 + 7 + 8 — — —— — — 5 1 + 2 7 + 8 — — — — — — 6 1 8 — — — — — — 7 1 + 8 — — — — — — —8 1 — — — — — — —

In some cases, the number of row indices for each table (where there isa table for each number of normal CSI-RS antenna ports) may be the same.This allows the field size for the indication of the row index forsupplemental CSI-RS ports and port mapping to be constant regardless ofthe number of normal CSI-RS antenna ports.

As described, UEs that use both normal and supplemental CSI-RS may havemore frequent CSI on the antenna ports that are contained in the normaland supplement antenna port sets than those UEs that use normal CSI-RSantenna ports only. UEs using both the normal and supplemental CSI-RSmay need to perform linear operations on the measurements obtained fromthe normal CSI-RS antenna ports to properly match the spatialorientation of the supplemental CSI-RS antenna ports. UEs usingsupplemental CSI-RS ports may feedback channel information according tothe format of the supplemental CSI-RS antenna ports.

PDSCH RE muting as described above may be used for the normal CSI-RSsubframe/RB. Alternatively, PDSCH REs corresponding to supplementalCSI-RS's from neighbor cells are not muted as CoMP operations based onshort term channel conditions may not be supported for higher speedmobiles. In these cases, the relative overhead associated with thesupplemental CSI-RS in comparison to the normal CSI-RS is small.

The number of RE's per RB per antenna port or virtual antenna stream maybe different for normal CSI-RS and supplemental CSI-RS due to differentconstraints on the reliability of CSI. Similarly, the periodicity of thesupplemental CSI-RS subframe may be variable. In some cases, there mayexist additional parameters needed by the UE for proper operation usingthe supplemental CSI-RS such as a supplemental CSI-RS format. Theparameters and periodicity of the supplemental CSI-RS may be indicatedin the SIB in a broadcast manner, or sent to a UE in a unicast ormulticast manner as needed.

FIG. 19 illustrates a wireless communications system including anembodiment of UE 10. UE 10 is operable for implementing aspects of thedisclosure, but the disclosure should not be limited to theseimplementations. Though illustrated as a mobile phone, the UE 10 maytake various forms including a wireless handset, a pager, a personaldigital assistant (PDA), a portable computer, a tablet computer, alaptop computer. Many suitable devices combine some or all of thesefunctions. In some embodiments of the disclosure, the UE 10 is not ageneral purpose computing device like a portable, laptop or tabletcomputer, but rather is a special-purpose communications device such asa mobile phone, a wireless handset, a pager, a PDA, or atelecommunications device installed in a vehicle. The UE 10 may also bea device, include a device, or be included in a device that has similarcapabilities but that is not transportable, such as a desktop computer,a set-top box, or a network node. The UE 10 may support specializedactivities such as gaming, inventory control, job control, and/or taskmanagement functions, and so on.

The UE 10 includes a display 702. The UE 10 also includes atouch-sensitive surface, a keyboard or other input keys generallyreferred as 704 for input by a user. The keyboard may be a full orreduced alphanumeric keyboard such as QWERTY, Dvorak, AZERTY, andsequential types, or a traditional numeric keypad with alphabet lettersassociated with a telephone keypad. The input keys may include atrackwheel, an exit or escape key, a trackball, and other navigationalor functional keys, which may be inwardly depressed to provide furtherinput function. The UE 10 may present options for the user to select,controls for the user to actuate, and/or cursors or other indicators forthe user to direct.

The UE 10 may further accept data entry from the user, including numbersto dial or various parameter values for configuring the operation of theUE 10. The UE 10 may further execute one or more software or firmwareapplications in response to user commands. These applications mayconfigure the UE 10 to perform various customized functions in responseto user interaction. Additionally, the UE 10 may be programmed and/orconfigured over-the-air, for example from a wireless base station, awireless access point, or a peer UE 10.

Among the various applications executable by the UE 10 are a webbrowser, which enables the display 702 to show a web page. The web pagemay be obtained via wireless communications with a wireless networkaccess node, a cell tower, a peer UE 10, or any other wirelesscommunication network or system 700. The network 700 is coupled to awired network 708, such as the Internet. Via the wireless link and thewired network, the UE 10 has access to information on various servers,such as a server 710. The server 710 may provide content that may beshown on the display 702. Alternately, the UE 10 may access the network700 through a peer UE 10 acting as an intermediary, in a relay type orhop type of connection.

FIG. 20 shows a block diagram of the UE 10. While a variety of knowncomponents of UEs 110 are depicted, in an embodiment a subset of thelisted components and/or additional components not listed may beincluded in the UE 10. The UE 10 includes a digital signal processor(DSP) 802 and a memory 804. As shown, the UE 10 may further include anantenna and front end unit 806, a radio frequency (RF) transceiver 808,an analog baseband processing unit 810, a microphone 812, an earpiecespeaker 814, a headset port 816, an input/output interface 818, aremovable memory card 820, a universal serial bus (USB) port 822, ashort range wireless communication sub-system 824, an alert 826, akeypad 828, a liquid crystal display (LCD), which may include a touchsensitive surface 830, an LCD controller 832, a charge-coupled device(CCD) camera 834, a camera controller 836, and a global positioningsystem (GPS) sensor 838. In an embodiment, the UE 10 may include anotherkind of display that does not provide a touch sensitive screen. In anembodiment, the DSP 802 may communicate directly with the memory 804without passing through the input/output interface 818.

The DSP 802 or some other form of controller or central processing unitoperates to control the various components of the UE 10 in accordancewith embedded software or firmware stored in memory 804 or stored inmemory contained within the DSP 802 itself. In addition to the embeddedsoftware or firmware, the DSP 802 may execute other applications storedin the memory 804 or made available via information carrier media suchas portable data storage media like the removable memory card 820 or viawired or wireless network communications. The application software maycomprise a compiled set of machine-readable instructions that configurethe DSP 802 to provide the desired functionality, or the applicationsoftware may be high-level software instructions to be processed by aninterpreter or compiler to indirectly configure the DSP 802.

The antenna and front end unit 806 may be provided to convert betweenwireless signals and electrical signals, enabling the UE 10 to send andreceive information from a cellular network or some other availablewireless communications network or from a peer UE 10. In an embodiment,the antenna and front end unit 806 may include multiple antennas tosupport beam forming and/or multiple input multiple output (MIMO)operations. As is known to those skilled in the art, MIMO operations mayprovide spatial diversity which can be used to overcome difficultchannel conditions and/or increase channel throughput. The antenna andfront end unit 806 may include antenna tuning and/or impedance matchingcomponents, RF power amplifiers, and/or low noise amplifiers.

The RF transceiver 808 provides frequency shifting, converting receivedRF signals to baseband and converting baseband transmit signals to RF.In some descriptions a radio transceiver or RF transceiver may beunderstood to include other signal processing functionality such asmodulation/demodulation, coding/decoding, interleaving/deinterleaving,spreading/despreading, inverse fast Fourier transforming (IFFT)/fastFourier transforming (FFT), cyclic prefix appending/removal, and othersignal processing functions. For the purposes of clarity, thedescription here separates the description of this signal processingfrom the RF and/or radio stage and conceptually allocates that signalprocessing to the analog baseband processing unit 810 and/or the DSP 802or other central processing unit. In some embodiments, the RFTransceiver 808, portions of the Antenna and Front End 806, and theanalog base band processing unit 810 may be combined in one or moreprocessing units and/or application specific integrated circuits(ASICs).

The analog base band processing unit 810 may provide various analogprocessing of inputs and outputs, for example analog processing ofinputs from the microphone 812 and the headset 816 and outputs to theearpiece 814 and the headset 816. To that end, the analog base bandprocessing unit 810 may have ports for connecting to the built-inmicrophone 812 and the earpiece speaker 814 that enable the UE 10 to beused as a cell phone. The analog base band processing unit 810 mayfurther include a port for connecting to a headset or other hands-freemicrophone and speaker configuration. The analog base band processingunit 810 may provide digital-to-analog conversion in one signaldirection and analog-to-digital conversion in the opposing signaldirection. In some embodiments, at least some of the functionality ofthe analog base band processing unit 810 may be provided by digitalprocessing components, for example by the DSP 802 or by other centralprocessing units.

The DSP 802 may perform modulation/demodulation, coding/decoding,interleaving/deinterleaving, spreading/despreading, inverse fast Fouriertransforming (IFFT)/fast Fourier transforming (FFT), cyclic prefixappending/removal, and other signal processing functions associated withwireless communications. In an embodiment, for example in a codedivision multiple access (CDMA) technology application, for atransmitter function the DSP 802 may perform modulation, coding,interleaving, and spreading, and for a receiver function the DSP 802 mayperform despreading, deinterleaving, decoding, and demodulation. Inanother embodiment, for example in an orthogonal frequency divisionmultiplex access (OFDMA) technology application, for the transmitterfunction the DSP 802 may perform modulation, coding, interleaving,inverse fast Fourier transforming, and cyclic prefix appending, and fora receiver function the DSP 802 may perform cyclic prefix removal, fastFourier transforming, deinterleaving, decoding, and demodulation. Inother wireless technology applications, yet other signal processingfunctions and combinations of signal processing functions may beperformed by the DSP 802.

The DSP 802 may communicate with a wireless network via the analogbaseband processing unit 810. In some embodiments, the communication mayprovide Internet connectivity, enabling a user to gain access to contenton the Internet and to send and receive e-mail or text messages. Theinput/output interface 818 interconnects the DSP 802 and variousmemories and interfaces. The memory 804 and the removable memory card820 may provide software and data to configure the operation of the DSP802. Among the interfaces may be the USB interface 822 and the shortrange wireless communication sub-system 824. The USB interface 822 maybe used to charge the UE 10 and may also enable the UE 10 to function asa peripheral device to exchange information with a personal computer orother computer system. The short range wireless communication sub-system824 may include an infrared port, a Bluetooth interface, an IEEE 802.11compliant wireless interface, or any other short range wirelesscommunication sub-system, which may enable the UE 10 to communicatewirelessly with other nearby mobile devices and/or wireless basestations.

The input/output interface 818 may further connect the DSP 802 to thealert 826 that, when triggered, causes the UE 10 to provide a notice tothe user, for example, by ringing, playing a melody, or vibrating. Thealert 826 may serve as a mechanism for alerting the user to any ofvarious events such as an incoming call, a new text message, and anappointment reminder by silently vibrating, or by playing a specificpre-assigned melody for a particular caller.

The keypad 828 couples to the DSP 802 via the interface 818 to provideone mechanism for the user to make selections, enter information, andotherwise provide input to the UE 10. The keyboard 828 may be a full orreduced alphanumeric keyboard such as QWERTY, Dvorak, AZERTY andsequential types, or a traditional numeric keypad with alphabet lettersassociated with a telephone keypad. The input keys may include atrackwheel, an exit or escape key, a trackball, and other navigationalor functional keys, which may be inwardly depressed to provide furtherinput function. Another input mechanism may be the LCD 830, which, mayinclude touch screen capability and also display text and/or graphics tothe user, The LCD controller 832 couples the DSP 802 to the LCD 830.

The CCD camera 834, if equipped, enables the UE 10 to take digitalpictures. The DSP 802 communicates with the CCD camera 834 via thecamera controller 836. In another embodiment, a camera operatingaccording to a technology other than Charge Coupled Device cameras maybe employed. The GPS sensor 838 is coupled to the DSP 802 to decodeglobal positioning system signals, thereby enabling the UE 10 todetermine its position. Various other peripherals may also be includedto provide additional functions, e.g., radio and television reception.

FIG. 21 illustrates a software environment 902 that may be implementedby the DSP 802. The DSP 802 executes operating system drivers 904 thatprovide a platform from which the rest of the software operates. Theoperating system drivers 904 provide drivers for the UE hardware withstandardized interfaces that are accessible to application software. Theoperating system drivers 904 include application management services(“AMS”) 906 that transfer control between applications running on the UE10. Also shown in FIG. 21 are a web browser application 908, a mediaplayer application 910, and Java applets 912. The web browserapplication 908 configures the UE 10 to operate as a web browser,allowing a user to enter information into forms and select links toretrieve and view web pages. The media player application 910 configuresthe UE 10 to retrieve and play audio or audiovisual media. The Javaapplets 912 configure the UE 10 to provide games, utilities, and otherfunctionality. A component 914 might provide functionality describedherein.

The UE 10, base station 120, and other components described above mightinclude a processing component that is capable of executing instructionsrelated to the actions described above. FIG. 22 illustrates an exampleof a system 1000 that includes a processing component 1010 suitable forimplementing one or more embodiments disclosed herein. In addition tothe processor 1010 (which may be referred to as a central processor unit(CPU or DSP), the system 1000 might include network connectivity devices1020, random access memory (RAM) 1030, read only memory (ROM) 1040,secondary storage 1050, and input/output (I/O) devices 1060. In somecases, some of these components may not be present or may be combined invarious combinations with one another or with other components notshown. These components might be located in a single physical entity orin more than one physical entity. Any actions described herein as beingtaken by the processor 1010 might be taken by the processor 1010 aloneor by the processor 1010 in conjunction with one or more componentsshown or not shown in the drawing.

The processor 1010 executes instructions, codes, computer programs, orscripts that it might access from the network connectivity devices 1020,RAM 1030, ROM 1040, or secondary storage 1050 (which might includevarious disk-based systems such as hard disk, floppy disk, or opticaldisk). While only one processor 1010 is shown, multiple processors maybe present. Thus, while instructions may be discussed as being executedby a processor, the instructions may be executed simultaneously,serially, or otherwise by one or multiple processors. The processor 1010may be implemented as one or more CPU chips.

The network connectivity devices 1020 may take the form of modems, modembanks, Ethernet devices, universal serial bus (USB) interface devices,serial interfaces, token ring devices, fiber distributed data interface(FDDI) devices, wireless local area network (WLAN) devices, radiotransceiver devices such as code division multiple access (CDMA)devices, global system for mobile communications (GSM) radio transceiverdevices, worldwide interoperability for microwave access (WiMAX)devices, and/or other well-known devices for connecting to networks.These network connectivity devices 1020 may enable the processor 1010 tocommunicate with the Internet or one or more telecommunications networksor other networks from which the processor 1010 might receiveinformation or to which the processor 1010 might output information.

The network connectivity devices 1020 might also include one or moretransceiver components 1025 capable of transmitting and/or receivingdata wirelessly in the form of electromagnetic waves, such as radiofrequency signals or microwave frequency signals. Alternatively, thedata may propagate in or on the surface of electrical conductors, incoaxial cables, in waveguides, in optical media such as optical fiber,or in other media. The transceiver component 1025 might include separatereceiving and transmitting units or a single transceiver. Informationtransmitted or received by the transceiver 1025 may include data thathas been processed by the processor 1010 or instructions that are to beexecuted by processor 1010. Such information may be received from andoutputted to a network in the form, for example, of a computer databaseband signal or signal embodied in a carrier wave. The data may beordered according to different sequences as may be desirable for eitherprocessing or generating the data or transmitting or receiving the data.The baseband signal, the signal embedded in the carrier wave, or othertypes of signals currently used or hereafter developed may be referredto as the transmission medium and may be generated according to severalmethods well known to one skilled in the art.

The RAM 1030 might be used to store volatile data and perhaps to storeinstructions that are executed by the processor 1010. The ROM 1040 is anon-volatile memory device that typically has a smaller memory capacitythan the memory capacity of the secondary storage 1050. ROM 1040 mightbe used to store instructions and perhaps data that are read duringexecution of the instructions. Access to both RAM 1030 and ROM 1040 istypically faster than to secondary storage 1050. The secondary storage1050 is typically comprised of one or more disk drives or tape drivesand might be used for non-volatile storage of data or as an over-flowdata storage device if RAM 1030 is not large enough to hold all workingdata. Secondary storage 1050 may be used to store programs that areloaded into RAM 1030 when such programs are selected for execution.

The I/O devices 1060 may include liquid crystal displays (LCDs), touchscreen displays, keyboards, keypads, switches, dials, mice, track balls,voice recognizers, card readers, paper tape readers, printers, videomonitors, or other well-known input/output devices. Also, thetransceiver 1025 might be considered to be a component of the I/Odevices 1060 instead of or in addition to being a component of thenetwork connectivity devices 1020. Some or all of the I/O devices 1060may be substantially similar to various components depicted in thepreviously described drawing of the UE 10, such as the display 702 andthe input 704.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods may beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

Also, techniques, systems, subsystems and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component, whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and may be made without departing from the spirit and scopedisclosed herein.

To apprise the public of the scope of this invention, the followingclaims are made:

1. A method of operating a network element in a wireless communicationsnetwork, the method comprising: identifying, by at least a networkelement, a channel state information reference signal (CSI-RS) that isorthogonal to CSI-RSs transmitted by each of a first network cell and aset of neighbor cells that interfere with the first network cell,wherein the first network cell has a coverage containing a coverage of asecond network cell, wherein the network element is associated with thesecond network cell; and transmitting, by the network element, theidentified CSI-RS.
 2. The method of claim 1, wherein the set ofinterfering neighbor cells is a subset of a CSI-RS network cell group,and the CSI-RS transmitted by the network element is the same as aCSI-RS transmitted by a network cell that is a member of the CSI-RSnetwork cell group, but not a member of the set of interfering neighborcells.
 3. The method of claim 2, wherein the CSI-RS network cell groupconsists of the first network cell, the second network cell, and the setof interfering neighbor cells.
 4. The method of claim 1, wherein the setof interfering neighbor cells is a subset of a CSI-RS network cell groupand the CSI-RS transmitted by the network element is orthogonal to eachof the CSI-RSs transmitted by each network cell of the CSI-RS networkcell group.
 5. The method of claim 4, wherein the CSI-RS network cellgroup consists of the first network cell and the set of interferingneighbor cells.
 6. The method of any of claims 1 to 5, wherein thesecond network cell is a small cell including at least one of a femtocell, a relay cell, or a pico cell.
 7. The method of any of claims 1 to6, wherein the first network cell is a macro cell.
 8. The method of anyof claims 1 to 7, wherein the coverage of the first network cellincludes at least a portion of a coverage of a third network cell, and:when the second network cell is moving, the CSI-RS transmitted by thesecond network cell is orthogonal to a CSI-RS transmitted by the thirdnetwork cell; when the coverage of the second network cell overlaps thecoverage of the third network cell, the CSI-RS broadcast by the secondnetwork cell is orthogonal to the CSI-RS transmitted by the thirdnetwork cell; and when the second network cell and the third networkcell are stationary and the coverage of the second network cell does notoverlap the coverage of the third network cell, the CSI-RS broadcast bysecond network cell is not orthogonal to the CSI-RS transmitted by thethird network cell.
 9. The method of claim 8, wherein the CSI-RSbroadcast by second network cell is the same as the CSI-RS transmittedby the third network cell.
 10. The method of any of claims 1 to 9,wherein the set of interfering neighbor cells is empty.
 11. A basestation for use in a wireless communication network, the base stationcomprising a processor configured to perform the method of any of claims1 to
 10. 12. The base station of claim 11, wherein the set ofinterfering neighbor cells is a subset of a CSI-RS network cell group,and the CSI-RS transmitted by the second network cell is the same as aCSI-RS transmitted by a network cell that is a member of the CSI-RSnetwork cell group, but not a member of the set of interfering neighborcells.
 13. The base station of claim 12, wherein the CSI-RS network cellgroup consists of the first network cell, the second network cell, andthe set of interfering neighbor cells.
 14. The base station of claim 11,wherein the set of interfering neighbor cells is a subset of a CSI-RSnetwork cell group and the CSI-RS transmitted by the second network cellis orthogonal to each of the CSI-RSs transmitted by each network cell ofthe CSI-RS network cell group.
 15. The base station of claim 14, whereinthe CSI-RS network cell group consists of the first network cell and theset of interfering neighbor cells.
 16. The base station of claim 11,wherein the second network cell is a small cell including at least oneof a femto cell, a relay cell, and a pico cell.
 17. The base station ofclaim 11, wherein the first network cell is a macro cell.
 18. The basestation of claim 11, wherein the coverage of the first network cellincludes at least a portion of a coverage of a third network cell, and:when the second network cell is moving, the CSI-RS transmitted by thesecond network cell is orthogonal to a CSI-RS transmitted by the thirdnetwork cell; when the coverage of the second network cell overlaps thecoverage of the third network cell, the CSI-RS broadcast by the secondnetwork cell is orthogonal to the CSI-RS transmitted by the thirdnetwork cell; and when the second network cell and the third networkcell are stationary and the coverage of the second network cell does notoverlap the coverage of the third network cell, the CSI-RS broadcast bysecond network cell is not orthogonal to the CSI-RS transmitted by thethird network cell.
 19. The base station of claim 18, wherein the CSI-RSbroadcast by second network cell is the same as the CSI-RS transmittedby the third network cell.
 20. The base station of claim 11, wherein theset of interfering neighbor cells is empty.
 21. A user equipment,comprising: a processor configured to communicate with a memory, thememory storing instructions, which when executed by the processor, causethe processor to perform the steps of: receiving, from a networkelement, a channel state information reference signal (CSI-RS) that isorthogonal to CSI-RSs transmitted by each of a first network cell and aset of neighbor cells that interfere with the first network cell,wherein the first network cell has a coverage containing a coverage of asecond network cell, and the network element is associated with thesecond network cell.
 22. The user equipment of claim 21, wherein the setof interfering neighbor cells is a subset of a CSI-RS network cellgroup, and the CSI-RS received from the network element is the same as aCSI-RS transmitted by a network cell that is a member of the CSI-RSnetwork cell group, but not a member of the set of interfering neighborcells.
 23. The user equipment of claim 21, wherein the set ofinterfering neighbor cells is a subset of a CSI-RS network cell groupand the CSI-RS received from the network element is orthogonal to eachof the CSI-RSs transmitted by each network cell of the CSI-RS networkcell group.